Copolymers of vinylidene fluoride and hexafluoropropylene having reduced extractable content and improved solution clarity

Abstract

New and novel copolymers of vinylidene fluoride and hexafluoropropylene containing up to 24% by weight hexafluoropropylene having lower DSC melting temperatures at 8 weight percent or greater nominal HFP content, and having improved solution clarity and fluidity, longer gel times and lower extractables than prior art vinylidene fluoride-hexafluoropropylene copolymers of comparable HFP content whose syntheses are disclosed in sufficient detail to duplicate, to novel compositions of matter and articles of manufacture containing such copolymers, processes for the preparation and use of the copolymers, of the compositions of matter containing such copolymers and of the articles of manufacture containing such copolymers are disclosed. Improved electrochemical cells based on the new and novel copolymers are particularly disclosed.

Description

This invention relates to compositions of matter classified in the art of chemistry as fluoropolymers, more specifically as copolymers of vinylidene fluoride (VDF), more specifically as copolymers of vinylidene fluoride and hexafluoropropylene (HFP), still more specifically as copolymers of VDF and HFP having reduced extractable content, longer gel times and improved solution clarity relative to VDF, HFP copolymers known previously which otherwise have similar proportions of HFP in the bulk polymer, to novel compositions of matter and articles of manufacture containing such copolymers, as well as to processes for the preparation and use of the copolymers, of compositions of manufacture containing such copolymers and of the articles of manufacture containing such copolymers.VDF / HFP copolymers are well known and are used for their thermoplastic engineering properties, chemical resistance and inertness toward degradation. They may be found in applications such as chemically resistant p...

AI Technical Summary

Benefits of technology

Pianca et al. in Polymer, vol. 28, 224-230 (February 1987) examined the microstructure of VDF / HFP copolymers by nuclear magnetic resonance, and the microstructure determinations were used to explain the trend in glass transition temperatures of the copolymers. The synthesis of the copolymers involved a semicontinuous emulsion process which used different VDF / HFP ratios for the initial fill of the reactor and for the subsequent continuous feed of monomers. No detailed synthesis examples were provided, and there was no discussion of copolymers having lowered melting points, improved solution clarity, longer gel times and low extractables as provided by the copolymers of the present invention.

None of these references teaches or suggests a way to obtain VDF / HFP copolymers having lower melting temperatures (when the copolymer has greater than 8 wt. % HFP content), solvent solution clarity and fluidity, longer gel times and low extractables when compared to the previously known VDF / HFP copolymers described by such references all of which have comparable freedom from extraneous impurities.

It has also been noted that batteries fabricated from the above described VDF polymers, particularly the VDF-HFP copolymers of the present invention have better adhesion of the polymers to metallic portions of electrodes and higher use temperatures than batteries fabricated from VDF-HFP copolymers of the prior art. It has also been observed that VDF-HFP copolymers of the present invention provide batteries having improved electrical properties including the capability of higher discharge rates than batteries fabricated from VDF-HFP copolymers of the prior art. It is expected by the present inventors that in general batteries fabricated according to the present invention will possess such higher temperature use and higher discharge rate capabilities.

The invention provides in a third composition aspect, a solution of a composition of the first composition aspect of the invention having improved solution clarity and fluidity.

Copolymers of vinylidene fluoride and hexafluoropropylene of up to about 24 wt % hexafluoropropylene are useful semicrystalline thermoplastics. As the HFP content increases in the materials, the crystallinity decreases, and, correspondingly, the flexibility and solvent sensitivity increase. Other properties change as well, such as the final melting point, which decreases with increasing HFP content. In high-purity applications such as membrane filtration or extraction, lithium battery construction, high-transparency film from solution casting, and fluid storage and transport requiring low contaminant levels, it is desirable to have materials with low levels of extractables, little gel formation in the presence of solvent, and good clarity. The VDF / HFP copolymers provided here show lower extractables, improved solution properties, improved clarity and fluidity, and lower melting points in comparison to the nonuniform VDF / HFP copolymers of otherwise similar HFP content whose manufacture is enabled in the prior art.

By uniform it is meant that there is decreased chain to chain variation in the distribution of HFP residues in polymer chains in comparison to the nonuniform copolymers of the prior art where the HFP content from chain to chain varies broadly, thereby imparting measurably different inherent physical and chemical properties.

Problems solved by technology

While a low conversion batch process is theoretically capable of producing copolymers having lower melting points, solution clarity and low extractables, no such properties are described.

It is not a practical process for industrial use because of the low conversions required to make the materials.

In addition, since no detailed polymerization examples were offered, it is not possible to reproduce the polymers tested with any degree of certainty.

It is preferred to use physical blowing agents in the manufacture of electrochemical cells since trace amounts of the chemical blowing agents in the foam structure could adversely affect the functioning of the cell.

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